This invention relates to a method of accelerating the setting time of concretes, mortars, and other compositions containing portland cement (i.e. "portland cement compositions") and to the use as accelerators of certain .alpha.-hydroxy carbonyl compounds, primarily hydroxyacetic (glycolic) acid, lactic acid, and certain salts of hydroxyacetic, lactic, 2-methyllactic, and dl-mandelic acids, in concentrations relative to the cement within ranges as specified herein.
Additives for modifying the setting time and other properties of portland cement compositions have been employed virtually since the beginning of cement technology in the 1870's. Cement itself, a mixture of (1) a lime-containing material such as limestone, shell or chalk, and (2) clay or a source of clay such as shale or slate, flash sets in a matter of seconds or minutes when mixed with water, leaving virtually no time for adequate mixing, pouring, or other necessary processing operations. To make it commercially useful, therefore, essentially all cement as sold contains a few percent gypsum, which delays setting for a number of hours. In order to shorten or lengthen setting time for specific applications, accelerating or retarding admixtures are employed. Such admixtures are defined by ASTM publication C 494-71, which also defines "water reducing" admixtures as those which reduce the amount of water necessary to produce concrete of a given consistency. Both accelerating and retarding admixtures are known which also effect water reduction.
The setting times for commercially available cements vary fairly widely, but are typically on the order of three hours as determined by the final setting time of the Vicat Needle method (ASTM C 191). The setting times of mortars made with such cements measured by the initial setting time of the Proctor Needle test (ASTM C 403) is about seven hours. (As is widely recognized in this art, both the Vicat Needle and Proctor Needle tests may be used to determine setting times of various portland cement compositions of various proportions, e.g. water-cement ratios, and are not restricted to the specific compositions and proportions of ingredients recited in ASTM C 191 and ASTM C 403. As used herein, the terms "Vicat Needle test" and "Proctor Needle test" refer to carrying out generally the procedures specified in the ASTM references on the particular sample of portland cement composition of interest.) These setting times must be significantly reduced, for example, in the manufacture of molded concrete products such as concrete blocks, in the manufacture of pavements and bride surfaces, and in the construction of concrete buildings. In numerous applications, the impetus for shortening the setting time is at least in part the high cost of overtime labor.
One application which inherently demands very short setting times is shotcreting, a process of spraying mortar on a supporting surface to obtain both structural strength and desired surface properties. Shotcreting is employed for example in the construction of tunnels drilled through rock, and the concrete must achieve a high percentage of its ultimate strength after it is sprayed on in a shorter period than the relaxation time of the rock in order to prevent the tunnel from collapsing.
There are both so-called wet and dry shotcrete processes. In the wet process, the cement mixture including water is fully prepared and then sprayed on the tunnel wall or other surface from a nozzle. In the dry process, all the dry ingredients are mixed and fed to the nozzle, and the water is added in the nozzle. Note that greater acceleratiion of setting time can be achieved generally with the dry process because there is no risk of the mixture setting up in the equipment prior to being sprayed. In either process the shotcrete must harden in less than fifteen minutes as measured by the Proctor Needle test final setting time in order that a useful thickness of shotcrete may be applied to a surface without its sloughing off.
Accelerating admixtures for shotcreting in general commercial use today are mixtures of sodium carbonate and sodium aluminate. While they afford acceptable early strengths relative to plain mortar, they typically reduce the ultimate strength of the mortar by more than half, requiring the use of thicker layers of mortar than would be necessary in the absence of the accelerator to achieve the same structural strength. Additionally, these admixtures are highly caustic and great care must be taken in handling them. Only a limited degree of acceleration can be achieved with sodium carbonate-sodium aluminate accelerators without reducing the ultimate strength of the shotcrete to an unacceptable level. There exist applications requiring shorter setting times and rapid early strength development for which no satisfactory accelerating admixtures have been available.
A second application which requires very fast setting times is the use of portland cement plugging compounds. Plugging compounds consist of portland cement, fine aggregate such as mason's sand, and an accelerator and are used to repair broken or cracked concrete when water is leaking through the fracture. Plugging compounds must have a final setting time as measured by the Vicat Needle test of less than fifteen minutes in order to ensure that the compound will harden and stop the leak before it is washed away.
Portland cement compositions used to repair damaged concrete articles and structures must harden quickly so that the time taken to make the repair will be short. Quick-setting-mortar compounds generally consist of portland cement, fine aggregate, and an accelerator and are used for repairing chipped or cracked concrete and other small jobs. They require final setting times of thirty minutes or less as measured by the Vicat Needle method. Concrete patching compounds contain cement, coarse and fine aggregate, and an accelerator and are generally used for making larger repairs such as filling potholes in concrete pavement. Concrete patching compounds must have final setting times of one hour or less as measured by the Vicat test; the setting times typically being somewhat longer than the setting times of quick-setting-mortar compounds because of the longer time required to mix and apply the larger quantities of concrete patching compound. The Vicat Needle test may be carried out directly on samples of mortar. In the case of concrete, the Vicat test is carried out on the mortar of the sample, which may be separated from the coarse aggregate by sieving, as provided in ASTM C 403.
Perhaps the most widely used accelerator for most concrete applications is calcium chloride. Although it does not accelerate setting times sufficiently for use in shotcrete processes, it is used for most other applications and has the advantage of being very inexpensive. It is severely limited, however, in that chloride ion is highly corrosive in contact with ferrous metals, and also promotes an electrochemical reaction between dissimilar metals. Hence calcium chloride generally cannot be used as an accelerator for reinforced concrete or other applications involving concrete-metal interfaces.
It has been known since the late 1930's that certain organic hydroxy carbonyl compounds could influence the setting times of portland cement, but in a manner precisely opposite that contemplated by this invention. The prior art teaches that certain hydroxy carbonyl compounds, including some of the .alpha.-hydroxy carbonyl compounds of the present invention, retard the setting of portland cement rather than accelerate it. This prior art, the most pertinent of which is discussed below, does not disclose or suggest the accelerators of the present invention and in fact teaches away from the present invention. In spite of the widely-recognized drawbacks of the accelerators in use today, it has not been recognized that the .alpha.-hydroxy carbonyl compounds of the present invention function as suitable accelerators for the applications disclosed even though certain of these compounds are disclosed in the prior art as admixtures for portland cement.
A review of the prior art relating to accelerators in general and to hydroxy-carbonyl retarders demonstrates that it is difficult to make generalizations about the effect of broad classes of compounds on the setting times of portland cement. For example, although the essentially ionic salt calcium chloride is an efficient accelerator, sodium chloride and potassium chloride, also ionic chlorides, are substantially less efficient accelerators. Moreover merely because a particular compound functions as an accelerator does not imply that there are concentrations at which it will shorten setting times sufficiently to be used in applications such as shotcrete and plugging compounds which require particularly short setting times. As noted above, for example, calcium chloride is a widely used accelerator for portland cement, but it does not shorten setting times sufficiently to be used for shotcreting.
One reference teaches broadly that the .alpha.-hydroxy carbonyl group, ##STR1## is very active in retarding the hydration of portland cement, the degree of hydration being a measure of its degree of hardening. In this reference; Proceedings of the International Symposium on the Chemistry of Cement, Washington, 1960, pages 924-925; J. H. Taplin classifies hydroxyacetic acid as a strong retarder on the basis of data obtained from cement paste specimens having water-cement ratios of 0.30 and containing one percent admixture by weight. Lactic acid, classified as having a negligible retarding effect, is called exceptional in that it contains the .alpha.-hydroxy carbonyl group but does not retard. This observation by Taplin indicates the difficulty of making broad generalizations concerning the effects of classes of compounds on the setting times of portland cement. Taplin nonetheless concludes that "it appears to be a general rule, that for an organic substance to retard cement, it must have at least two oxygen atoms each bound to a single but different carbon atom in such a way that the oxygen atoms can approach each other." The Taplin reference contains no teaching or suggestion that hydroxyacetic or lactic acid might be useful as an accelerator for portland cement compositions and plainly leads away from the present invention.
A second reference which similarly teaches away from the present invention is U.S. Pat. No. 3,144,347, issued Aug. 11, 1964 to E. I. du Pont de Nemours & Company. The '347 patent teaches the use of hydroxyacetic acid, lactic acid, and their sodium, calcium, potassium and amine salts as retarding admixtures in concentrations ranging from about 0.001 to 3.5 weight percent relative to the cement. The patent recites further (Column 1, lines 33-35) that "most often less than about 0.8 weight percent will be used, and particularly advantageous results are obtained within the narrow range of 0.01 to 0.1 weight percent", and all of the specific examples contained in columns 3 and 4 of the patent relate to the use of less than 0.8 weight percent of the foregoing retardant admixtures in concretes. The second paragraph of the patent defines the scope of the asserted invention as relating to "a small class of monohydroxy organic compounds . . . which not only serve as excellent water reducing retardants but at the same time causes a significant increase in the compressive strength of the concrete." The sole and plain teaching of the '347 patent is thus that the recited compounds are useful as retarding agents and concomitantly as ultimate strength increasing agents.
The only suggestion whatsoever in the prior art that a specific .alpha.-hydroxy carbonyl compound may have some utility as an accelerator is found in Japanese Patent Application Publication 13680 of 1972, dated Apr. 24, 1972 and assigned to Hani Chemical Company, Ltd., by Kenji Harazawa. Precisely what the Harazawa patent does or does not teach one of ordinary skill in this art is at best unclear. It states that calcium monglycolate and calcium diglycolate effect a limited reduction in the initial setting time of portland cement, and further that "it is particularly preferred to use portland cement and mixed gypsum plaster." I am not sure what "gypsum plaster" is, or why one would want to mix it with cement. Although the terminology is ambiguous, calcium diglycolate is generally used to refer to the calcium salt of diglycolic acid, which is not an .alpha.-hydroxy carbonyl compound. Even giving Harazawa the benefit of all doubts, his preferred range is between 0.05 and 0.15 percent admixture with respect to the cement, and the shortest final setting time disclosed for either compound is one hour and forty-five minutes and the shortest final setting time for calcium monoglycolate is one hour and fifty minutes, as measured by JIS R-5201, a Vicat Needle test. For reasons stated above, it is not possible to extrapolate with respect to the effect of higher concentrations of admixture, and Harazawa contains no suggestion whatsoever that the salts recited by him, or any other compounds, might make useful accelerators for applications requiring fast setting times such as shotcreting, plugging compounds, and quick-setting mortars. The patent contains no suggestion of the invention disclosed and claimed herein.